The invention relates generally to oscillator circuits and, in particular, to an oscillator circuit that is capable of starting and operating at low voltages with few components and, thus, consume less power.
Oscillator circuits are used in a myriad of applications in the electronics industry for providing clock and other timing signals to electronic circuitry such as microprocessors, microcontrollers, flip-flop circuits, latch circuits, etc. Oscillator circuits typically generate an electronic signal that oscillates at some predetermined frequency between a first voltage level and a second voltage level. The first and second voltage levels are typically determined by the level of the supply voltage applied to the circuit, while the predetermined frequency is typically determined by the resonant frequency of a resonant circuit such as a crystal, resonator or series RLC circuit.
One common way to build an oscillator circuit is to couple a resonant circuit across an inverter circuit. Several types of inverter structures may be used to accomplish the inversion function such as a complementary push-pull type inverter. These inverter types are typically used in low power applications because they require about one-half the power of non push-pull inverters, and do not experience the high frequency limitation of non push-pull inverters.
Referring to FIG. 1, a prior art implementation of an oscillator circuit (10) including a push-pull inverter circuit (12) coupled across a resonant circuit (14) is shown. Resonant circuit 14 may take the form of a resonator, a crystal or a series resistor-inductor-capacitor (RLC) circuit such that resonant circuit 14 resonates (i.e., oscillates) at a predetermined frequency. In particular, resonant circuit 14 may take the form, for example, of a resonator having part number CSA 4.00 MG manufactured by muRata Erie.
Oscillator circuit 10 also includes a large value resistor 15, coupled across resonant circuit 14, for setting the quiescent condition during start-up and normal operation. Resistor 15 may have a value, for example, in the range of 1 Megohm to 12 Megohms.
Inverter circuit 12 includes a driver stage comprising P-channel transistor 18 and N-channel transistor 20. The gate/control electrodes of each are coupled to a first terminal of resonant circuit 14, while the drain electrodes (first current carrying electrode) of each are coupled to a second terminal of resonant circuit 14 and to output terminal 16 at which an output oscillatory signal is provided. The source electrode (second current carrying electrode) of transistor 18 is coupled to a first supply voltage terminal at which the operating potential V.sub.DD is applied, while the source electrode of transistor 20 is coupled to a second supply voltage terminal at which the operating potential V.sub.SS is applied.
Generally, resonant circuit 14 amplifies signals appearing at the output of inverter 12 that are at its resonant frequency and supplies these signals back to the input of inverter 12. Inverter 12 provides the 180 degree phase shift and gain required to begin and maintain oscillation. When functioning properly, oscillator circuit 10 provides an output signal at terminal 16 that oscillates at the resonant frequency of resonant circuit 14 while swinging between the voltage ranges of V.sub.DD and V.sub.SS. As an example, if voltage V.sub.DD was 5 volts, voltage V.sub.SS was 0 volts, and the resonant frequency of resonant circuit 14 was 10 MHz, the signal appearing at terminal 16 would be an oscillatory (i.e., sinusoidal-type) signal swinging from approximately 0 to 5 volts at a frequency of 10 MHz.
However, for proper start-up, both transistors 18 and 20 must be maintained active (i.e., on) which requires that the minimum voltage between V.sub.DD and V.sub.SS be at least the sum of the threshold voltages of transistors 18 (V.sub.TP18) and 20 (V.sub.TN20), as represented in EQN. 1. EQU V.sub.DD -V.sub.SS &gt;V.sub.TP18 +V.sub.TN20 EQN. 1
As an example, if threshold voltages V.sub.TP18 and V.sub.TN20 are both equal to 1.5 volts, then the voltage difference between the supply voltages must be at least 3 volts, plus a few tenths of a volt for overhead, to provide for proper start-up. This minimum voltage assures that each transistor remains active and that the voltage appearing at the gate electrodes of transistors 18 and 20 does not float. Therefore, if the voltage difference falls below 3 volts, then at least one of the transistors will be inactive and oscillator circuit 10 will not initiate oscillation from power-up. Such a minimum voltage requirement, however, conflicts with the general trend of reducing the operating supply voltages for electronic circuits since reducing voltages achieves many advantages such as longer battery life, use of fewer batteries and overall less power consumption.
Thus, it is a principal object of the present invention to provide an improved inverter circuit for use in an oscillator circuit that is capable of starting-up and operating with low supply voltages and, thus, consuming less power.